The present invention relates to a seabed mound creation method in the technology for constructing a gravity-type large offshore structure on a soft ground in a shallow sea area where the depth of water is about 10 to 30 m, and more particularly to an improved method of creating a seabed mound having a high stability to the ground in an offshore or an ocean area where supply of mound materials is difficult.
When constructing a gravity-type offshore structure, the shallower the base position is, the more economical the structure is. For this reason, creation of seabed mounds is ordinarily carried out. In most cases, such mounds are formed by heaping up mound materials e.g., riprap or sands and so forth on the seabed.
However, when the seabed consists of soft or weak ground, mound materials cannot be heaped up thereon without taking additional measures. To overcome this, in the prior art, there has been employed a method as disclosed in Japanese patent application No. 58-69878, wherein the method comprises the steps of improving the soft ground on a seabed, heaping up sands and rocks on the improved ground to form a seabed mound, and setting up a structure on the seabed mound thus formed.
For this reason, construction of the mound must be executed in two working steps for ground improvement of the seabed and for the mound construction, resulting in a large number of working steps and prolonged term of construction. Further, it takes much time to construct a mound and increase the cost of transport in an offshore or an ocean area where the supply of mound materials is limited.
Meanwhile, when an offshore structure is constructed in a sea area where an earthquake is likely to happen, in which there exists the strong influence of a seismic force as an external force, or in a sea area where there exists the strong influence of an external force due to waves or tides, a considerably large horizontal external force acts on the offshore structure. On the other hand, so called friction-type structures are constructed so as to remove the horizontal external force by making use of the frictonal resistance force between the improved ground and the mound materials heaped up thereon and the frictional resistance force between the mound and the offshore structure. However, with such friction-type structures, it is quite difficult to ensure a safety factor of the recent design standard.
Namely, in the case of the gravity-type offshore structure, the study of the stability between the mound and the structure will be made based on the following equation, EQU F.sub.S =W' .mu./F.sub.H &gt;F.sub.SO,
where F.sub.S is a safety factor, W' is a weight when buoyancy is taken into account, .mu. is a friction coefficient, F.sub.H is a horizontal external force, and F.sub.SO is a specified safety factor.
As is clear from the above equation, if a horizontal external forth F.sub.H due to an earthquake is excessive, the resistance due to the friction coefficient .mu. is limited. In many cases, this makes it difficult to guarantee a sufficiently large safety factor F.sub.S.